Single layer fuel lines and vapor return lines of synthetic materials such as polyamides have been proposed and employed in the past. Fuel lines employing such materials generally have lengths of at least several meters. In order to fulfill their function, the material employed in the fuel line must be inert to the medium flowing through them and resistant to high and low temperatures and to mechanical loads. Once installed, it is important that the line not materially change during the length of operation either by shrinkage or elongation or as a result of the stresses to which the line may be subject during use.
Single layer tubes are not always capable of meeting the necessary requirements. Transport of materials containing aliphatic or aromatic chemicals can create undesirable disadvantages such as hydrocarbon permeation, dimensional changes or insufficient mechanical load bearing capacity.
It has become increasingly important that fuel lines be essentially impervious to hydrocarbon emissions due to hydrocarbon permeation through the tube itself. The permissible hydrocarbon emissions due to permeation through fuel lines is severely limited by federal and state regulations. Regulations in states such as California fix the total passive hydrocarbon emission for a vehicle at 2 g/m.sup.2 per 24 hour period as calculated by evaporative emission testing methods outlined in Title 13 of the California Code of Regulations, Section 1976 as amended Sep. 26, 1991. To achieve the desired total vehicle emission levels, a hydrocarbon permeation level for fuel lines equal to or below 0.5 g/m.sup.2 per 24 hour period is desirable. It is also necessary that the fuel line employed be impervious to interaction with corrosive materials present in the fuel such as oxidative agents and surfactants as well as additives such as methanol and ethanol.
In the past, various types of tubing have been proposed to address these concerns. Co-extruded multi-layer tubing has met with various levels of success. Such multi-layer tubing generally employs a relatively thick outer layer composed of a material resistant to the exterior environment. The innermost layer (or layers) is thinner and is composed of a material which is chosen for its ability to block diffusion of materials, such as aliphatic hydrocarbons, alcohols and other materials present in fuel blends, to the outer layer.
Alcohol and aromatic components in the fuel conveyed through the tube tend to diffuse through the tubing wall at rates different from any diffusion of aliphatic fuel components. It was feared that the resulting change in the composition of the liquid in the tubing would alter solubility thresholds of the tubing material to a degree sufficient to be able to crystalize monomers and oligomers of materials such as nylon 12 and nylon 11 into the liquid. It was feared that crystallized nylon precipitate would block fuel filters, fuel injectors and the like and would collect to limit the travel of the fuel pump or carburetor float as well as build up on critical control surfaces of the fuel pump.
In order to avoid this phenomenon, patents such as U.S. Pat. No. 5,076,329 to Brunhoffer disclosed a five-layer fuel line in which the inner fluid-contacting surface was composed of nylon 6. In U.S. Pat. No. 5,038,833 to Brunhoffer, a three layer fuel line was proposed in which a tube is formed having a co-extruded outer wall of nylon 11 or nylon 12, an intermediate alcohol barrier wall formed of ethylene vinyl alcohol copolymer and an inner water blocking wall formed of a polyamide such as nylon 11 or nylon 12. In DE 40 06 870, a fuel line was proposed in which an intermediate solvent barrier layer is formed of unmodified nylon 6.6 either separately or in combination with polyamide elastomers. The innermost layer is also composed of polyamides such as modified or unmodified nylon 6 while the outer layer is composed of either nylon 6 or nylon 12. UK 2 204 376A discloses a tube which has a thick outer layer composed of a polyamide such as nylon 6 or nylon 6.6 and/or nylon 11 or nylon 12 coextruded with an alcohol-resistant polyolefin, and a copolymer of propylene and maleic acid.
In certain situations, it has been found that multi-layer tube having an innermost layer composed of nylon 11 or nylon 12 can be employed without adverse effects. In U.S. Pat. No. 5,313,987, a multi-layer pipe is disclosed which has an outer polyamide layer and an intermediate layer formed from a mixture of a thermoplastic polyester such as polybutylene terepthalate and a compound having at least two isocyanate groups. The innermost layer may, optionally, be composed of a polyamide.
In all instances, it is necessary to provide proper uniform laminar adhesion between all layers of the multi-layer tubing while achieving appropriate permeation resistance. Thus, advances and innovations regarding bonding materials which can be successfully employed in permeation resistant multi-layer tubing would be highly desirable.
In many instances, it is also desirable to provide a tube which can address the phenomenon of the build up of electrostatic charge on the surfaces of the tubing body. Electrostatic charge is defined as electric charge on the surface of a body. In this instance, a fuel tube, which occurs when charges are created at a rate faster than they can be dissipated. A net charge imbalance results. The charge imbalance will continue to increase until a limiting event such as a spark discharge occurs. In multi-layer fuel tubing, the electric charge built up is derived from the passage of charged particles through tubing constructed from essentially nonconductive materials.
Spark discharges generally occur in one of three ways: (a) directly to ground (through air); (b) from one part of a component to another (through air); or (c) to ground through the walls of a component (by dielectric breakdown). If a spark discharge due to dielectric breakdown occurs, the energy released in the discharge event can melt, degrade or burn the polymeric material in the discharge path and produce a pinhole rupture. Once dielectric breakdown occurs, the insulative properties of the material are weakened and subsequent breakdowns will generally occur at lower electrical field strength. Any subsequent discharge or current flow will tend to occur along the more conductive path already created thereby enlarging the hole or rupture. If this occurs in multi-layer fuel tubing, the risk of tubing rupture is increased with associated increases in the danger of fire and/or explosion of flammable contents in the tubing.
Various patents discussed the problems of lamination strength, permeation resistance, and electrostatic discharge prevention. Among these are U.S. Pat. No. 5,383,087 to Noone et al which discloses a multi-layer polymeric tube having at least four layers in which an inner layer and an innermost electrostatic discharge layer preferably composed of a fluoroplastic are bonded to an outer layer composed of a thermoplastic material such as thermoplastic elastomers and polyamides. The bonding layer is preferably composed of thermoplastic made up of a polyvinyl fluoride compound and a polyamide.
U.S. Pat. No. 5,524,673 also to Noone et al, discloses various multi-layer tubing constructions having at least three layers in which an outer layer is composed of a thermoplastic material selected from the group consisting of thermoplastic elastomers, polyamides, and mixtures thereof. The inner fuel-contacting layers disclosed in this reference include fluoropolymers selected from the group consisting of polyvinylidine fluoride, polyvinyl fluoride, polychlorotrifluoroethylene, ethylene tetrafluoroethylene copolymers, graft copolymers of the preceding fluoroplastic materials together with a fluorine-containing compound such as copolymers of vinylidine fluoride and chlorotrifluoroethane. Alternately, the inner fuel-contacting layer can be composed of a thermoplastic material selected from the group consisting of thermoplastic elastomers, polyamides selected from the group consisting of nylon 12, nylon 11, nylon 6, and mixtures thereof. The materials specified for the intermediate bonding layer include certain fluoroplastic materials such as polyvinylidene fluoride and polyvinyl fluoride; non-fluoroplastics such as ethylene vinyl alcohol and polyvinyl acetate-urethane blends; and thermoplastic polyesters such as polybutylene terepthalate as well as the various blends discussed in Noone '087. In the multi-layer tubing disclosed in U.S. Pat. No. 5,524,673, one of the layers may optionally be capable of dissipating electrostatic energy.
U.S. Pat. No. 5,460,771 to Mitchell et al discloses a process for making corrugated multi-layer tubing in which the outer layer can be made from any number of specified thermoplastic materials while the inner fuel contacting layer is made from either a fluoroplastic material or a material which is chemically similar to the outer layer. Bonding layers are composed of the materials previously enumerated in connection with Noone '673. U.S. Pat. No. 5,566,720 to Mitchell et al discloses a multi-layer tube in which the bonding layer is a terpolymer containing a polyfluorinated alkylene, an .alpha.-fluoro-olefin and a fluorinated vinyl compound.
The use of fluorinated materials such as ethylene chlorotrifluoroethylene copolymers and polyvinylidine fluoride copolymers has accomplished many of the objectives such as reduced hydrocarbon permeation through the tube. However, the need exists for multi-layer tube constructions which achieve hydrocarbon permeation resistance at levels equal to or greater than those achieved by multi-layer tubing constructions containing fluoropolymers. Additionally, the need exists for multi-layer tube constructions which exhibit effective inter-layer lamination and prolonged service life.
It is desirable to provide a multi-layer tube suitable for use in motor vehicles which is durable and can prevent or reduce permeation of organic materials therethrough. It is desirable to provide a multi-layer tube material in which the various layers are uniformly connected to one another in a manner which will prevent or reduce the potential for delamination between layers during the life of the tubing. Finally, it would be desirable to provide a multi-layer tube which would be capable of preventing undesired build up of electrostatic charge and would be capable of safely dissipating any electrostatic charge which may occur.